1. Field of the Invention
The present invention relates to a structure of an air bag door portion and a method of producing the air bag door portion.
2. Description of the Conventional Art
In recent years, in order to protect occupants of a motor carat the time of collision, as shown in FIG. 17 and FIG. 18 showing a section taken in line C--C in FIG. 17, an air bag device 90 has been provided on the back of an instrument panel 60. The air bag device 90 includes an air bag case 91 secured to e.g. reinforcement member of a car body and disposed on the back of the instrument panel 60, an inflator (operating device) 92 accommodated within the air bag case 91 and a folded air bag 93. On the other hand, the instrument panel 60 is provided with an air bag door portion 61 immediately above the air bag device 90. As seen from FIG. 19, when the inflator 92 is operated because of shock at the time of collision, the air bag 93 expands to push the air bag door portion 61 from the back into a car chamber or compartment. Thus, the air bag door 61 is opened so that the air bag 93 develops into the compartment 93.
There have been proposed two kinds of instrument panels having the air bag door portion. In the one kind of instrument panel, a separate door portion is attached after the instrument panel is molded. In another kind of instrument panel, the air bag door portion is formed integrally to the instrument panel while it is manufactured. From the viewpoint of manufacturing workability and appearance of the edge of the air bag door portion, the integrally molded product is preferred.
The instrument panel shown in FIGS. 18 and 19 is an example in which it is integrally provided with the air bag door portion 61. The instrument panel 60 is structured as follows. A door substrate (base material) 64 of hard resin or metal is disposed in an air bag door opening 63 formed in an instrument panel substrate (base material) 62 of e.g. hard resin, so that the one side 65 which is the side of a hinge H of the door substrate 64 is secured at the peripheral edge of the air bag door opening 68 by a securing member 66 composed of a bolt and nut. In addition, a foam member 68 is foam-molded between a surface member 67 and the door substrate 64 and the instrument panel substrate 62 at the peripheral edge thereof. On the remaining side 69 of the door substrate 64, a securing piece 70 is provided which is secured to the peripheral edge of the air bag door opening 63. On the surface member 67, a breakage or rupture scheduling portion 71, which defines the position of the air bag door portion 61 and is broken at the time of expansion of the air bag, is formed fragilely as a notch. The portion which serves as the hinging side H of the door substrate 64 at the peripheral edge of the air bag door opening 63 undergoes excess force at the time of expansion of the air bag. Therefore, the peripheral edge of the air bag door opening 63 is sandwiched by a bracket 72 which is secured to an air bag case 91 by welding so that it is reinforced. Reference numeral 73 denotes another securing member and 74 denotes another bracket.
The air bag door portion 61 is adapted so that when the securing piece 70 of the door substrate 64 is pushed from its back because of the expansion of the air bag 93, the securing piece 70 of the door substrate 64 comes off from the peripheral edge of the air bag door opening 63. Then, the foam member 68 and the breakage scheduling portion 71 of the surface member 67 are broken along the peripheral edge of the door substrate 64. As a result, the air bag door portion 61 is bent and opened into the vehicle chamber at a fulcrum of the hinge H side of the door substrate. Accordingly, the air bag 93 can develop into the vehicle chamber.
The air bag door portion 61 is manufactured as seen from FIG. 20. After the door substrate 64 is attached to the air bag door opening 63 of the instrument panel substrate 62, the instrument panel substrate 62, door substrate 64 and surface member 67 are disposed in the cavity 76 in a foaming mold 75. Between the door substrate 64 and instrument panel substrate 62, and the surface member 67, liquid foaming raw material 77 such as polyurethane is injected to mold the foam member 68. In this case, in order to prevent the foaming material 77 from being leaked, the peripheral edge of the door substrate is brought into contact with the peripheral edge of the air bag door opening 63 of the instrument panel substrate 62.
Meanwhile, as seen from FIG. 21 showing a section taken in line D--D in FIG. 18, in order that the integrally-molded air bag door portion 61 can be opened at the time of expansion of the air bag 93, it is structured so that on both sides of the door substrate 64 in a direction of a vehicle width, the peripheral edge 78 of the air bag door opening of the instrument panel substrate 62 is only secured to the securing pieces 70 at the peripheral edge of the door substrate, but not secured to the air bag case 91. Therefore, the door substrate 64 is likely to deform as indicated by arrow by pushing force when the air bag 93 expands. Further, as described above, when the air bag door portion is molded, the peripheral edge of the door substrate 64 is kept in intimate contact with the peripheral edge 78 of the air bag door opening so that the foaming material does not leak. For this purpose, a large number of the securing pieces 70 are provided at the peripheral edge of the door substrate 64, which are engaged with the peripheral edge 78 of the air bag door opening. Therefore, when the air bag 93 expands, the edge except the hinging side of the door substrate 64 is difficult to separate smoothly from the air bag door opening. Further, in the integral molding of the air bag door portion, the foam member 68 is foam-molded as an integral product in which the air bag door portion and its surrounding general portion are continuous so that when the air bag 93 expands, the foam member 68 must be broken forcibly at the peripheral edge of the door substrate. Thus, when the air bag 93 expands, the air bag door portion 61 and its peripheral portion first deform greatly Next, the securing pieces 70 are separated and foam member 68 is broken. Finally, the air bag door portion 61 is opened. Accordingly, the instrument panel may be damaged or broken owing to such deformation.
In order to prevent the instrument panel from being deformed or damaged owing to direct application of the pushing force to the instrument panel when the air bag expands, the air bag door portion 86 can be structured as shown in FIG. 23 using the door substrate 80 as shown in FIG. 22. Specifically, securing pieces 81 are provided at the peripheral edge of the door substrate to secure an instrument panel substrate 82 to the peripheral edge 83 of the air bag door opening. On the other hand, on the back of the door substrate 80, the securing piece 84 which protrudes along the side of an air bag case 91a is provided to secure the air bag case 91a to the securing piece 34. In addition, on the back of the door substrate 80, a fragile portion for breakage 85 may be formed along the inside of another securing piece 84b except the securing piece 84a on the hinging side Ha. In this case, the fragile portion 85 for breakage of the door substrate 80 is broken when the air bag 93 expands. Thus, with the securing piece 84b on the outer periphery of the broken portion and its outside being left on the peripheral edge of the air bag opening 83, the air bag door portion 86 is opened into the compartment at a fulcrum of the hinge siding side H. Therefore, the pushing force when the air bag 93a expands is applied to the inside of the door substrate 80 mainly encircled by the fragile portion 85 so that the deforming force applied to the peripheral edge 83 of the air bag door opening of the instrument panel substrate 82 can be reduced.
In the structures as shown in FIGS. 22 and 23, the fragile portion 85 for breakage is provided along the inside of the securing piece 86. Therefore, when the surface of the air bag door portion 86 is pushed by an occupant or others, the stress is concentrated on the fragile portion 85 so that the fragile portion may be easily broken and the surface of the air bag door portion 86 may sink. This impairs the appearance of the instrument panel. Incidentally, in order to prevent the fragile portion 85 for breakage from being broken owing to the pushing force from the surface side of the air bag door portion 86, a soft material with low bending resiliency and low rigidity may be used as the door substrate 80. However, in order to prevent the sink due to the breakage, the bending resiliency of the door substrate 80 must be reduced to 2000-3000 kg/cm.sup.2 which is extremely lower than the ordinary bending resiliency of 7000 kg/cm.sup.2 The material having the low bending resiliency is too soft so that a large difference in the hardness is produced between the door substrate 80 and its peripheral hard instrument panel 82. As a result, a difference in the hardness occurs between the surface of the air bag door portion 86 and that of its surrounding general portion. This gives a sense of incongruity in touch of the surface.